This type of a flow rate measurement device of a fluid in the related art is generally configured as illustrated in a configuration diagram of FIG. 5 (for example, see PTL 1).
The flow rate measurement device includes first ultrasonic wave oscillator 122 and second ultrasonic wave oscillator 123 installed in flow path 121 through which a fluid flows, and switching unit 124 for switching transmission and reception of first ultrasonic wave oscillator 122 and second ultrasonic wave oscillator 123. In addition, the flow rate measurement device includes transmitting unit 125 for driving first ultrasonic wave oscillator 122 and second ultrasonic wave oscillator 123, amplifier 126 that amplifies a received signal which is received by an ultrasonic wave oscillator on a receiving side and passes through switching unit 124 to a predetermined amplitude, and reference comparator 127 for comparing a voltage of the received signal amplified by amplifier 126 with a reference voltage.
The flow rate measurement device includes determination unit 128 that detects zero-cross point a of the received signal after amplified received signal A is compared with reference voltage Vr by reference comparator 127 and the received signal becomes larger than reference voltage Vr as illustrated in an explanatory view of FIG. 6. Furthermore, the flow rate measurement device includes time measuring unit 129 that measures a propagation time of transmission and reception of the ultrasonic waves from timing detected by determination unit 128, and controller 130 that performs control of transmitting unit 125 and amplifier 126, and calculates a flow speed and/or a flow rate based on a time measured by time measuring unit 129.
Transmitting unit 125 is operated by controller 130 and an ultrasonic wave signal transmitted by first ultrasonic wave oscillator 122 propagates in the flow, is received by second ultrasonic wave oscillator 123, and is amplified by amplifier 126, and then is signal-processed by reference comparator 127 and determination unit 128, and is input into time measuring unit 129.
Next, first ultrasonic wave oscillator 122 and second ultrasonic wave oscillator 123 are switched by switching unit 124, and the same operation is performed, so that respective propagation times of a fluid to be measured from an upstream side to a downstream side (assuming the direction to be a forward flow) and from the downstream side to the upstream side (assuming the direction to be a reverse flow) are measured by time measuring unit 129.
Here, a flow rate Q can be obtained by the following equation in which an effective distance between the ultrasonic wave oscillators in a flowing direction is L, the propagation time from the upstream side to the downstream side is t1, the propagation time from the downstream side to the upstream side is t2, the flow speed of the fluid to be measured is v, a cross-sectional area of the flow path is S, and a sensor angle is ϕ.Q=S·v=S·L/2·cos ϕ(n/t1−n/t2)  (Equation 1)
Actually, a flow rate is calculated by further multiplying a coefficient corresponding to the flow rate to Equation 1.
In addition, an amplification rate of amplifier 126 is adjusted such that controller 130 adjusts an amplification degree to cause the signal received by the ultrasonic wave oscillator (first ultrasonic wave oscillator 122 or second ultrasonic wave oscillator 123) on the receiving side to have a constant amplitude, and a maximum voltage value of the received signal falls within a predetermined voltage range.
During the measurement, in a case where the maximum voltage value of the received signal falls below a lower limit (voltage range lower limit) of the predetermined voltage range as illustrated by received signal b in a broken line of an explanatory view of FIG. 7 or exceeds an upper limit (voltage range upper limit) of the predetermined voltage range as similarly illustrated by received signal c in a broken line of FIG. 7, the amplification rate at the next flow rate measurement is adjusted. For example, in a case where the maximum voltage value falls below the lower limit, the amplification degree is increased so that the maximum voltage value falls between the upper limit and the lower limit of the voltage range like received signal illustrated in a solid line of FIG. 7.
In addition, the reference voltage of reference comparator 127, which is compared with the received signal amplified by amplifier 126, determines the position of the zero-cross point detected by determination unit 128. Therefore, in FIG. 6, for example, the reference voltage is set to a voltage of a midpoint of peak voltages of a third wave and the fourth wave of the received signal when propagating in the air such that zero-cross point a of the fourth wave of the received signal is detected by determination unit 128.
By doing so, even if the peak voltage of the third wave increases or the peak voltage of the fourth wave decreases for some reason, a margin is obtained for both the peak voltage of the third wave and the peak voltage of the fourth wave and determination unit 128 can stably detect zero-cross point a of the fourth wave.
Moreover, in the configuration described in PTL 1, since the reference voltage is always a fixed value, for example, in order to stably detect zero-cross point a, as illustrated in FIG. 6, the reference voltage is set to the midpoint of the third wave peak and the fourth wave peak in which an interval of the peak voltages of the received wave is the widest when propagating in the air. However, in a case where a measurement object changes from the air to a gas other than the air, a received waveform may significantly change from the case of the air depending on the gas. As a result, there is a problem that in a case where the peak of the third wave of the received signal greatly increases, the zero-cross point of the third wave is erroneously detected, or in a case where the peak of the fourth wave of the received signal greatly decreases, a zero-cross point of a fifth wave is erroneously detected.
As a countermeasure against this, there is a method of changing the reference voltage according to the amplification degree in amplifier 126 so that the zero-cross point of the fourth wave can be stably measured with respect to various gases (for example, see PTL 2).